A total of 21 people were seen by medical staff following the suspected nerve agent attack.

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Detective Sergeant Nick Bailey rushed to the aid of a Russian ex-spy targeted with a nerve agent (Wiltshire Police/PA)

A police officer critically injured responding to the Salisbury nerve agent attack is in a stable condition, authorities confirmed as the injury toll climbed to 21.

Detective Sergeant Nick Bailey rushed to the aid of Russian double agent Sergei Skripal and his daughter Yulia, who had been struck down by a toxic substance.

Wiltshire Police temporary Chief Constable Kier Pritchard said the officer was in a “serious, but stable, condition” and had since regained consciousness.

(PA Graphics)

Both remain seriously ill in hospital following the murder bid on Sunday.

As uncertainty lingered over their fate, police said a total of 21 people had been seen for medical treatment in the aftermath of the incident.

The figure includes members of the public and emergency staff, some of whom have had blood tests as well as receiving support and advice.

It is feared that Mr Skripal – who was convicted in his home country for selling secrets to MI6 – was singled out for a state-sponsored assassination plot.

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Home Secretary Amber Rudd talks to Wiltshire Police Assistant Chief Constable Kier Pritchard in Salisbury, where Sergei Skripal and his daughter were found critically ill after exposure to a nerve agent (Andrew Matthews/PA)

He joined the force in 2002 and was commended in 2016 his work on a rape investigation.

Prime Minister Theresa May echoed the sentiment, saying the events served as a “stark reminder” of the dangers faced by emergency staff.

The acting Chief Constable of Wiltshire police says the officer injured in the Salisbury spy poisoning is sitting-up and recovering in hospital pic.twitter.com/4KiLkuiyqI

Mr Pritchard said in a statement: “I have visited Nick in hospital this afternoon and I can confirm that he is conscious, talking and engaging with people.

“He still remains in a serious, but stable, condition, but I am deeply relieved to have visited him and spoken with him in person.”

He added in an interview with Sky News: “Of course he’s very anxious, he’s very concerned. He did his very best on that night.”

It comes amid a flurry of activity in Salisbury, where police sealed off the gravestone of Mr Skripal’s wife Liudmila, who was buried in 2012, and the memorial stone of his son, Alexander, who was cremated last year.

They are located at separate sites in the London Road Cemetery, with each guarded by a police officer.

Hundreds of investigators, led by counter-terror police, are working to find out who is responsible for what is feared to be a sophisticated plot amid heightened tensions between Britain and Russia.

On Thursday night, an eyewitness said police had been searching a car at a vehicle recovery centre close to the Russian victim’s home earlier that day.

He described seeing around 10 people in hazmat suits looking at the vehicle, which was said to be a Mercedes people-carrier.

I want to express my heartfelt thanks to the emergency services for their bravery and professionalism in continuing to deal with the incident in Salisbury. My thoughts are with all those affected, including the police officer who is being treated in hospital.